This invention relates to the design of hydrostatic transmissions and integrated hydrostatic transaxles (IHT) and more particularly to an improved filter design for use with such transmissions and transaxles. The discussion of the invention herein will focus on an IHT as the preferred embodiment; however, it will be understood that this invention could be used with other hydrostatic designs as well.
Such an IHT will generally comprise a hydrostatic pump and motor mounted on a center section, where the hydraulic porting is formed into the body of the center section. It is generally understood that in an IHT the hydrostatic transmission and the gear elements are generally running in the same oil sump. This means such contaminants from the rotating gears, such as metal shavings, will be in the oil, and a filter is needed to prevent such contaminants from entering into the hydrostatic porting and damaging the running surfaces for the rotating cylinder blocks of the hydrostatic transmission. Check valves may be formed in the center section and opened at various times to permit oil to flow from the main oil sump into the hydraulic porting formed in the center section. It is generally known to provide a filter that covers these check valves or otherwise filters the oil before it can enter the check valves. One such known design is shown in U.S. Pat. No. 5,314,387 where the filter is a cap placed on the bottom of the center section, and the filter element is formed around the periphery thereof. A second  known design is shown in U.S. Pat. No. 6,185,936 where the filter element is on the bottom surface of the cap placed over the bottom of the center section.
It is also well known that excess air in an IHT system can cause noise and early failure of the hydrostatic elements. When an IHT is being assembled, the air is purged from the system, which means that all of the air in the center section and the hydrostatic porting therein is removed and replaced by hydraulic oil. There are occasions, however, for servicing of such an IHT unit in the field; during such servicing it is difficult to completely purge air from the IHT unit and in particular from the center section. Because of the geometry of the center section and oil filter, air may remain trapped therein after the completion of purging. The air trapped in the filter will be adjacent to the check valves, and thus there is risk that the air will be drawn into the center section through the check valves. Furthermore, air can coalesce from the oil during operation to form bubbles that can also become trapped adjacent to the check valves.